1. Field of the Invention
An object of the present invention is an X-ray image intensifier tube, especially of the type used in medicine, either in direct radioscopy or in radiology, with digitalized processing of the signal representing the image. The invention more particularly concerns a device to correct the distortion contributed to the image given with instruments of this type.
2. Description of the Prior Art
An X-ray image intensifier tube is designed to receive low-power X-radiation and convert this X-radiation into luminous radiation of higher power, which can be more easily detected by a display means, especially by a television camera. The reason why the X-radiation received is weak has to be related to the concern, especially in medicine, with protecting patients under examination by means of X-radiation of this type. This happens especially when these examinations are lengthy, as is the case with processing operations with digitization of information on image, or as may be the case with future-generation tomodensitometers where the detecting element will be precisely an X-ray image intensifier of this type.
An image intensifier tube essentially has a conversion panel to convert a received X-radiation into a luminous radiation capable of driving a photocathode, placed so as to face this panel. The conversion from X-radiation to light radiation is got in a known way by furnishing the panel with caesium iodide crystals. Under the effect of the illumination, photoelectrons are liberated from the photocathode and move towards a screen. This movement towards the screen is subjected to the action of an electronic lens. This electronic lens tends to make the impact of the photoelectrons on the screen correspond to the position of the photocathode from which they have been emitted. The screen itself is of a particular type. It re-emits a light image representing the electronic image conveyed by the electrons which themselves represent the X-ray image. This light image can then be detected by any display means, particularly by a target of a standard television camera. This light image is written on a face in front of the target while a target reading beam reads the written image on the other face.
A display sequence of this type has a major drawback: the image revealed is one that is geometrically distorted as compared with the X-ray image from which it originates. This distortion occurs essentially between the photocathode, excited by the photons coming from the conversion panel, and the screen which receives the electronic radiation emitted by the photocathode. For, while they are in transit, the photoelectrons are subjected to disturbing effects, especially magnetic effects caused by the earth's magnetic field. If the photoelectrons, while in transit, were all affected by one and the same type of disturbance, then correcting the effect of these disturbances at any point in the sequence of images would be enough to avoid any inconvenience therefrom. Unfortunately, these photoelectrons are highly sensitive, and the non-homogeneity of the magnetic field at the positions through which they pass is such that it results in a distortion of the electronic image projected on the screen. To explain the effects of a distortion of this type in more concrete terms, it can be said that the image of a straight line interposed between an X-ray tube and an image intensifier of this type will be a straight line in the X-ray image which excites the panel; it will be a straight line in the photonic image which drives the photocathode; it will be a straight line in the electronic image which leaves this photocathode, but will no longer be a straight line in the electronic image which gets displayed on the screen. Consequently, it can no longer be a straight line in the light image produced by this screen. The display device which is placed downline thus shows, somewhat, the result of the distortion due to the non-homogeneity of the earth's magnetic field in the space crossed by the electronic image.
Until now, this type of drawback could be overlooked because the images that were sought to be produced were essentially qualitative ones, and because little attention was paid to their quantitative content, namely the exactness with which the contours of the revealed objects were drawn. However, at present, with the development of new techniques, it is increasingly being sought to employ images of this type quantitatively. For example, it might be desired to perform prosthesis on the basis of the images obtained, and, in this case, distorted images would be intolerable. Moreover, in industrial checking, this type of fault makes it impossible to use images intensifiers of this type easily in metrology. In the same way, with future tomodensitometers, this deterioration will prevent accurate reconstruction of simultaneously acquired images of slices.
Various methods, essentially tending to modify the disturbing magnetic field, have been proposed to overcome these drawbacks. In a first set of methods, the image intensifier tube is encased with a magnetic shield. This shield channels the magnetic field lines and reduces the effects of the distortion. However, for reasons related to radiological absorption, a shield of this type cannot be placed above and in the vicinity of the external face of the conversion panel. Consequently, the magnetic distortions continue to exist near this panel. Unfortunately, precisely in the vicinity of this panel, the electrons liberated from the photocathode still have very low speeds. They are therefore very sensitive, at this location, to all the magnetic disturbances.
Besides, following the same line of thinking which led to the use of shields, it has also been proposed to place a magnetic field correction coil near the conversion panel. This coil is wound on the periphery of the panel. The French Patent No. 88 04071, filed on Mar., 29, 1988, even proposed setting up an automatic control link between the current flowing through this coil and a measurement of the component of the magnetic field colinear with the main axis of the image intensifier tube.
It might be thought that this latter technique could be extended to the measurement of the transverse components of the disturbing magnetic field so as to compensate for its effects. However, this method cannot be considered because the correction coils produce correcting magnetic fields that are independent of one another. These coils react on one another in such a way that the overall correction very quickly becomes inextricable. However, the need to take the distortions contributed by the transverse components of the magnetic field into account becomes vitally important inasmuch as it is sought to use the image intensifier tubes for purposes of morphometry. It is also possible to envisage the acquisition of a typical image, distorted by the disturbances, and the deduction therefrom of the corrections to be made to normal images, acquired under the same conditions as that of the typical image. The correction of the distortion in these normal images, based on mathematical algorithms applied by computer programs, is shown to be limited when the volume of information to be processed becomes great. For, this information on distortion is essentially related to the position of the image intensifier tube in space at the moment when it receives an X-radiation to be measured through an object to be X-rayed. Firstly, the very numerous positions possible for an image intensifier tube of this type makes for great bulk in the storage of this information on correction. Secondly, the application of the computed corrections to the normal images, calling for the use of bilinear algorithms (with multiplications), takes long to process if the number of correction bits is great. One method aimed at lightening the task of performing computations of this type consists in limiting the corrective magnitudes to be taken into account. Ultimately, it is sought to limit the number of computation bits. If the result of the computer program correction is considered to be a precise correction of the image distortion, a rough correction of this distortion must be got by other means.
An object of the present invention is to overcome these drawbacks by proposing a simple method which does not bring complicated arrangements of correction coils into play but contributes to a significant reduction in the number of processing bits to be managed. The invention is based on the following observation: the disturbing transverse components of the magnetic field have transverse effects in the created image. In this case, rather than seeking to counter the disturbing magnetic effects of the field, in their distortion of the writing on the television camera target, the method is confined to measuring the existence of these disturbing components and to taking them into account to organize the reading of this target of the television camera. In particular, with the horizontal scan and vertical scan controls of the beam for reading the target of this television camera, it is possible to take into account the original shift as well as modifications, if any, in the range of exploration of the target according to the measurements of these disturbing components.